1. Field of the Invention
The present invention relates to a metal oxide with piezoelectric characteristics having a novel composition. In particular, the present invention relates to a piezoelectric material containing neither lead nor an alkali metal and having a tungsten bronze structure.
2. Description of the Related Art
Piezoelectric materials are widely utilized in applications such as actuators, ultrasonic vibrators, ultrasonic motors, surface acoustic wave filters, and high voltage generators. The piezoelectric material mainly used in them is PZT [Pb(Zr,Ti)O3, lead zirconate titanate] which has a perovskite structure and contains lead. Thus, in view of environmental problems, lead-free piezoelectric materials have been developed.
A major reason why the above PZT is used widely is because a composition referred to as a morphotropic phase boundary (MPB) which markedly enhances the piezoelectric property is present. In the case of PZT, the crystal system takes a form of either tetragonal or rhombohedral structure depending on the ratio of zirconium and titanium, and their boundary in the crystal system is described to be generally near Ti/(Zr+Ti)=0.48. The piezoelectric constant of PZT increases several times near this morphotropic phase boundary. Therefore, in the development of the lead-free piezoelectric materials, it is important how a material system capable of forming a morphotropic phase boundary which enhances the piezoelectric property is developed.
In tungsten bronze structure oxides, a possibility has been shown that a morphotropic phase boundary is formed by mutually dissolving tungsten bronze structure oxides where symmetry of crystals is different even when the crystal systems are the same.
According to Journal of American Ceramic Society, Volume 72, pages 202 to 211 (1989), as a lead-free tungsten bronze MPB composition, (1-X)Ba2NaNb5O15—XSr2NaNb5O15 has been reported, but (1-X)Ba2NaNb5O15—XSr2NaNb5O15 contains an alkali metal.
WO2005/075378 describes a tungsten bronze structure oxide containing neither lead nor alkali metal, but there is no description on the crystal system and the MPB.
As described above, no tungsten bronze structure piezoelectric material, which has MPB and contains neither lead nor alkali metal, has been ever known.
Furthermore, WO2005/075378 describes a tungsten bronze type ferroelectric material represented by (Ba1−x−ySrxCay)2Ag1−dNb5O15−2/d (0.1≦x+y≦0.8, 0≦d≦0.6). In this case, when d is 0<d, the total formal charges of the cations is always less than +30. Thus, this is the structure in which the charges can not be balanced unless oxygen is always deficient.
When d is 0, the formula is (Ba1−x−ySrxCay)2AgNb5O15. This is obtained by substituting a part of Ba in Ba2AgNb5O15 with either one or more of Sr or Ca which has the same valence, and because of 0.1≦x+y≦0.8, the molar amount of Ba is always less than two times of Ag.
The conventional piezoelectric materials containing lead or alkali metals have two issues. One is the high environmental load attributed to the toxicity of lead, and the other issue is the troubles related with the alkali metals upon producing devices. Due to the high diffusion coefficient and high vapor pressure of the alkali metals, it is difficult to obtain piezoelectric ceramics with a high composition uniformity and a high density.